Current load device and method for driving the same

ABSTRACT

This invention provides a precise current load device. A cell includes a power supply line, a ground line, first and second voltage supply lines, a signal line, first, third and fourth control lines, first to fourth switches, a p-type TFT, a capacitance element, and a current load element. A source of the p-type TFT is connected to the power supply line, one terminal of the current load element is connected to the ground line, the first switch is connected between the signal line and a drain of the p-type TFT, the second switch is connected between the drain and the gate of the p-type TFT, the third switch is connected between the drain of the p-type TFT and the current load element, and the fourth switch is connected between the voltage supply line and the current load element.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a current load driving circuitfor driving a current load element and a method for driving the same. Inparticular, it relates to a current load device comprising current loadelements and current load driving circuits arranged in a matrix, and amethod for driving the same.

[0003] 2. Description of the Prior Art

[0004] In recent years, a device having cells arranged in a matrix, eachof the cells comprising a current load element that operates dependingon a current passing therethrough and a current load driving circuit fordriving the current load, has been developed.

[0005] For example, in many light emitting display devices with anorganic EL (electroluminescence) device serving as the current loadelement, pixels each comprising the organic EL device and a drivecircuit therefor are arranged in a matrix and driven according to theactive matrix method. FIG. 37 is a schematic plan view of a displayapparatus of such a light emitting display device. As shown in thisdrawing, on a display apparatus 1, there are formed a plurality ofcontrol lines CL extending in a row direction (the control lines areassigned consecutive numbers #1, #2, . . . , #(K−1), #K, #(K+1), . . . )and a plurality of signal lines SL extending in a column direction (thesignal lines are assigned consecutive numbers #1, #2, #(M−1), #M,#(M+1), . . . ). A pixel 2 is formed at an intersection of the controlline CL and the signal line SL. This display device is driven asfollows: the control lines CL are selected one by one; insynchronization with selection of one control line CL, the signal linesSL are supplied with brightness signals for pixels connected to theselected control line CL; in this state, the brightness signals arewritten to the pixels in the selected row; and the pixels continues theillumination according to the respective written signals until thecontrol line is selected again.

[0006] A typical configuration of the pixel of the light emittingdisplay device according to this method is shown in FIG. 38 (referred toas a first conventional example, hereinafter). As shown in FIG. 38, thesignal line SL (#M), a power supply line VCC, a ground line GND and thecontrol line CL (#K) pass through the pixel 2, and a light emittingdevice LED has an anode connected to the power supply line VCC and acathode connected to the drain of a TFT (thin film transistor) Q, andthe source of the TFT Q is connected to the ground line GND. A switchSW1 is connected between the gate of the TFT Q and the signal line SLand controlled by the control line CL. A capacitance element C isconnected between the gate of the TFT Q and the ground line GND.

[0007] An operation of the pixel according to this first conventionalexample is as follows. When the control line CL is selected, the switchSW is turned on. At this time, a voltage enough to supply a currentaccording to a current-brightness characteristic of the light emittingdevice LED is applied to the gate of the TFT Q through the signal lineSL so as to cause the light emitting device LED to emit light withbrightness at an intended gray-scale level. The gate voltage ismaintained (retained) by the capacitance element C, even when thecontrol line CL is deselected and the switch SW1 is turned off. Thisoperation enables the light emitting device LED to maintain brightnessat an expected gray-scale level.

[0008] The first conventional example has a disadvantage. That is, whenthere is un-unifomity in TFT's current/voltage characteristics, even ifa same voltage is applied to gates, the light emitting devices aresupplied with various currents. Consequently, the light emitting devicesare not supplied with a current enough to provide an expectedbrightness, and thus, the quality of the display device is reduced. Inparticular, there is quite large deviation of current/voltagecharacteristics of poly-silicon TFTs, which are often used in displaydevices, so that the image quality thereof is significantly reduced.

[0009] To solve the problem, there has been implemented a method ofsupplying a current to a transistor in the pixel circuit through thesignal line, converting the current into a voltage by the transistor andmaintaining (retaining) the voltage.

[0010]FIG. 39 is a circuit diagram showing an arrangement of the pixelof the light emitting display device according to the method ofsupplying a current signal through the signal line, which is disclosedin Japanese Patent Laid-Open No. 11-282419 (referred to as a secondconventional example, hereinafter). As shown in FIG. 39, a signal lineSL (#M), a power supply line VCC, a ground line GND and a control lineCL (#K) pass through a pixel 2. A light emitting device LED has an anodeconnected to the power supply line VCC and a cathode connected to thedrain of a TFT Q1, and the source of the TFT Q1 is connected to theground line GND. A switch SW1, which is controlled by the control lineCL, is connected between the signal line SL and the drain of a TFT Q2,and the TFT Q2 has the gate and the drain short-circuited and the sourceconnected to the ground line GND. A switch SW2, which is controlled bythe control line CL, is connected between the gate of the TFT Q1 and thegate of the TFT Q2. In addition, a capacitance element C is connectedbetween the gate of the TFT Q1 and the ground line GND.

[0011] An operation of the pixel according to this second conventionalexample is as follows. When the control line CL is selected, theswitches SW1 and SW2 are turned on. At this time, a current according toa current-brightness characteristic of the light emitting device LEDflows through the signal line SL to cause the light emitting device LEDto emit light with a brightness at an intended gray-scale level. Thiscurrent flows between the drain and source of the TFT Q2. However, sincethe gate and drain of the TFT Q2 are short-circuited, the gate voltagethereof is set at a value for passing the same current through the TFTQ2 in a saturation region, and the voltage is retained by thecapacitance element C. The TFT Q1 and the TFT Q2 form a current mirror.Thus, if current/voltage characteristics of TFT Q1 are equal to those ofthe TFT Q2, a current, whose value is equal to that of the currentflowing through TFTQ2 and the signal line SL, flows through the TFT Q1and is supplied to the light emitting device LED. Then, even if thecontrol line CL is deselected, the gate voltage of the TFT Q1 ismaintained (retained) by the capacitance element C. Therefore, the TFTQ1 can supply the current to the light emitting device LED, and thelight emitting device LED can maintain brightness at an expectedgray-scale level.

[0012]FIG. 40 is a circuit diagram of one pixel of another lightemitting display device according to the method of supplying a currentrequired for light emission with an intended brightness through thesignal line, which is disclosed in “Digest of IEDM” (1998), pp. 875-878by R. M. A. Dawson et al. As shown in FIG. 40, a pixel 2 of this lightemitting display device comprises a signal line SL (#M), a power supplyline VCC, a ground line GND, a control line CL1 (#K) and a control lineCL2 (#K) passing therethrough, four p-channel TFTs (p-TFT, hereinafter)Qp1 to Qp4, a light emitting device LED and a capacitance element C. Thep-TFT Qp4 has the gate connected to the control line CL2, the sourceconnected to the power supply line VCC and the drain connected to thesource of the p-TFT Qp1. The drain of the p-TFT Qp1, as well as thedrain of the p-TFT Qp3 having the gate connected to the control lineCL1, is connected to an anode of the light emitting device LED. Thesource of the p-TFT Qp3 is connected to the gate of the p-TFT Qp1, and acathode of the light emitting device LED is connected to the ground lineGND. The p-TFT Qp2 has the gate connected to the control line CL1, thesource connected to the signal line SL and the drain connected to thesource of the p-TFT Qp1 and the drain of the p-TFT Qp4. In addition, thecapacitance element C is connected between the gate and source of thep-TFT Qp1.

[0013] An operation of the pixel according to this third conventionalexample is as follows. If the pixel 2 is selected, the control line CL1(#K1) enters into an “L” state, the control line CL2 (#K) enters into an“H” state, the p-TFT Qp2 and the p-TFT Qp3 are turned on, and the p-TFTQp4 is turned off. Then, a current according to a current-brightnesscharacteristic of the light emitting device LED flows through the signalline SL (#M) to cause the light emitting device LED to emit light with abrightness at an intended gray-scale level. This current is supplied tothe light emitting device LED through the TFT Qp2 and TFT Qp1. At thistime, the p-TFT Qp1 has the drain and the gate short-circuited via thedrain and source of the p-TFT Qp3 and operates in the saturation state,the gate voltage of the p-TFT Qp1 is set at a value to provide thecurrent, and the voltage is retained by the capacitance element C. Whenthe selection of the control line shifts from the lines #K to the next,the control line CL1 (#K) enters into the “H” state, the control lineCL2 (#K) enters into the “L” state, and the supply of the current fromthe signal line SL to the pixel is stopped. However, the p-TFT Qp4 isturned on, and the current flows through this transistor. In this case,the gate voltage of the p-TFT Qp1, when the current from the signal lineSL flows through the p-TFT Qp1, is maintained (retained) by thecapacitance element C. Therefore, the p-TFT Qp1 can supply the currentto the light emitting device LED, and the light emitting device LED canmaintain a brightness at an expected gray-scale level.

[0014] According to the first conventional example described above, thebrightness depends on the voltage signal. However, there is quite largedeviation of current/voltage characteristics of poly-silicon TFTs, andeven if the same voltage is applied to the gates of TFTs, the lightemitting devices are supplied with various currents, and thus, thebrightness thereof varies. Therefore, there is a disadvantage that it isdifficult to cause the light emitting device to emit light with anintended brightness, and the quality of the display device is reduced.

[0015] According to the second conventional example, a pair oftransistors forming the current mirror are each constituted by a TFT.However, unlike with a crystalline silicon transistor, it is possiblethat the transistors of the pair have current/voltage characteristicswhich are significantly different from each other even when they aredisposed close to each other. Therefore, a difference in current/voltagecharacteristics appears between the transistor for retaining(converting) the current and the transistor for supplying the current tothe light emitting device, and thus, it becomes difficult to reproducean intended brightness with high precision.

[0016] In the case of the third conventional example described above, ifthe organic EL or the like is used as the light emitting device, thelight emitting device has a capacitance of the order of several pF inparallel therewith, and the capacitance constitutes a load on thedriving TFT. Thus, when a pixel is to be selected, it takes a long timefor the current value of the driving TFT to settle at a value forsupplying an expected current to the light emitting device and for thevoltages of the parts to settle in a state where the expected current issupplied to the light emitting device. Therefore, if the selectionperiod is shortened to accommodate higher definition, the selectionperiod will expire before the gate voltage of the p-TFT Qp1 settles at avalue at which the current flowing through the signal line equals to thecurrent the p-TFT Qp1 supplies to the light emitting device, and thus,the p-TFT Qp1 cannot supply an expected current to the light emittingdevice. Then, the light emitting device LED emits light with anunexpected brightness, and thus, the image quality is reduced. That is,the third conventional example has a disadvantage in that enhancing thedefinition reduces the image quality.

BRIEF SUMMARY OF THE INVENTION

[0017] This invention is to solve such problems of the prior art arisingin driving a current load element, and in particular, a light emittingdevice, such as an organic EL device. A first object of this inventionis to provide a current load device which can supply current loadelements with high precision. A second object thereof is to provide acurrent load device which can be increased in definition and sizewithout degradation in device characteristics by allowing a voltagebetween a source and gate of a driving TFT to quickly settle at a valuefor passing an expected current through the driving TFT.

[0018] In order to attain the objects, according to this invention,there is provided a current load device comprising: a driving transistorhaving a source connected to a power supply line or a ground line GNDdirectly or via a transistor; a first switch connected between a signalline and a drain of the driving transistor; a second switch connectedbetween the drain of the driving transistor or the signal line and agate of the driving transistor; a capacitance element having oneterminal connected to an appropriate voltage line and the other terminalconnected to the gate of the driving transistor; and aserially-connected assembly of a current load element and a thirdswitch, the serially-connected assembly being connected between theground line or any power supply line and the drain of the drivingtransistor.

[0019] Preferably, the third switch is turned on when the first switchis turned off, and is turned off before the first switch is turned on.More preferably, a fourth switch, which operates oppositely to the thirdswitch, is connected to the current load element in parallel.

[0020] In addition, in order to attain the objects, according to thisinvention, there is provided a method for driving a current load device,the current load device being active-matrix driven and comprising aplurality of cells each comprising a current load element, a drivingtransistor for supplying a driving current to the current load elementand a retention capacitance element for retaining a voltage to beapplied to the driving transistor, wherein a current is not supplied tothe current load element at least during a period in which the retentioncapacitance element conducts the retaining operation.

[0021] Preferably, the supply of the current to the current load elementis stopped before the retention capacitance element starts the retainingoperation. More preferably, when the supply of the current to thecurrent load element is stopped, charges stored in the current loadelement are forcedly removed.

[0022] [Operation]

[0023] According to the arrangement of this invention described above, aswitch is provided between the driving transistor for retaining andsupplying the current and the current load element, and the switch isheld in the of f state during a period of operation of retaining thecurrent; that operation is to set a gate voltage of the driving TFT toflow an appropriate current between the drain and the source of thedriving TFT via the signal line. Therefore, in retaining the current,the effect of the capacitance of the current load element can beeliminated and the current can be retained in a short time.

[0024] Besides, in the case of an arrangement in which the switch SWbetween the driving transistor for retaining and supplying the currentand the current load element is turned off an arbitrary time after thestart of the supply of the current to the current load element, theperformance of the current load element becomes the time-averageperformance controlled by the ratio of operating and un-operating periodof the current load element. In this case, to attain the sameperformance as in the case of not stopping the operation, theperformance of the current load element needs to be increased while itoperates and the current supplied to the current load element has to beincreased, so that the current supplied to the signal line is alsoincreased. Therefore, a time required for charging the capacitance ofthe signal line or load can be reduced, and a time required forretaining the current can be reduced.

[0025] In addition, if the current load element is a light emittingdevice, such as an organic EL device, since the display operationinvolves the state where the light emission is stopped as describedabove, the display operation is similar to that of CRTs (cathode raytubes) and an afterimage is hard to remain, and thus, moving images canbe displayed with higher quality.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1 is a diagram showing a configuration of a pixel accordingto the first embodiment of this invention;

[0027]FIG. 2 is a timing chart of an (first) operation example accordingto the first embodiment of this invention;

[0028]FIG. 3 is a timing chart of (second) another operation exampleaccording to the first embodiment of this invention;

[0029]FIG. 4 is a timing chart of (third) another operation exampleaccording to the first embodiment of this invention;

[0030]FIG. 5 is a diagram showing a configuration of a pixel accordingto the second embodiment of this invention;

[0031]FIG. 6 is a timing chart of an operation example according to thesecond embodiment of this invention;

[0032]FIG. 7 is a diagram showing a configuration of a pixel accordingto the third embodiment of this invention;

[0033]FIG. 8 is a diagram showing a configuration of a pixel accordingto the fourth embodiment of this invention;

[0034]FIG. 9 is a timing chart of an operation example according to thefourth embodiment of this invention;

[0035]FIG. 10 is a diagram showing a configuration of a pixel accordingto the first example of this invention;

[0036]FIG. 11 is a (first) diagram for illustrating an operationaccording to the first example of this invention;

[0037]FIG. 12 is a (second) diagram for illustrating an operationaccording to the first example of this invention;

[0038]FIG. 13 is a timing chart of an operation according to the firstexample of this invention;

[0039]FIG. 14 is a diagram showing a configuration of a pixel accordingto the second example of this invention;

[0040]FIG. 15 is a diagram showing a configuration of a pixel accordingto a third example of this invention;

[0041]FIG. 16 is a timing chart of an operation according to the thirdexample of this invention;

[0042]FIG. 17 is a diagram showing a configuration of a pixel accordingto the fourth example of this invention;

[0043]FIG. 18 is a timing chart of an operation according to the fourthexample of this invention;

[0044]FIG. 19 is a diagram showing a configuration of a pixel accordingto the fifth example of this invention;

[0045]FIG. 20 is a timing chart of an operation according to the fifthexample of this invention;

[0046]FIG. 21 is a timing chart of an operation according to a ninthexample of this invention;

[0047]FIG. 22 is a diagram showing a configuration of a pixel accordingto the tenth example of this invention;

[0048]FIG. 23 is a timing chart of an operation according to the tenthexample of this invention;

[0049]FIG. 24 is a diagram showing a configuration of a pixel accordingto the eleventh example of this invention;

[0050]FIG. 25 is a timing chart of an operation according to theeleventh example of this invention;

[0051]FIG. 26 is a diagram showing a configuration of a pixel accordingto the twelfth example of this invention;

[0052]FIG. 27 is a timing chart of an operation according to the twelfthexample of this invention;

[0053]FIG. 28 is a (first) diagram showing a configuration of a pixelaccording to the thirteenth example of this invention;

[0054]FIG. 29 is a timing chart of an operation according to thethirteenth example of this invention;

[0055]FIG. 30 is a (second) diagram showing a configuration of a pixelaccording to the thirteenth example of this invention;

[0056]FIG. 31 is a (first) diagram showing a configuration of a pixelaccording to the fourteenth example of this invention;

[0057]FIG. 32 is a (second) diagram showing a configuration of a pixelaccording to the fourteenth example of this invention;

[0058]FIG. 33 is a (first) diagram showing a configuration of a pixelaccording to the fifteenth example of this invention;

[0059]FIG. 34 is a timing chart of an operation according to thefifteenth example of this invention;

[0060]FIG. 35 is a (second) diagram showing a configuration of a pixelaccording to the fifteenth example of this invention;

[0061]FIG. 36 is a (third) diagram showing a configuration of a pixelaccording to the fifteenth example of this invention;

[0062]FIG. 37 is a schematic plan view of a display apparatus of a lightemitting display device;

[0063]FIG. 38 is a diagram showing a configuration of a pixel accordingto a first conventional example;

[0064]FIG. 39 is a diagram showing a configuration of a pixel accordingto a second conventional example; and

[0065]FIG. 40 is a diagram showing a configuration of a pixel accordingto a third conventional example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0066] Now, embodiments of the present invention will be described indetail with reference to the drawings. In the following, the descriptionwill be made with regard to a light emitting device. However, it isillustrative only, and this invention can be applied to any typicalcurrent load element.

[0067] [First Embodiment]

[0068]FIG. 1 is a circuit diagram showing a configuration of one pixelaccording to a first embodiment of this invention. As shown in FIG. 1, asignal line SL extending in a column direction, control lines CL1 to CL3extending in a row direction and voltage supply lines PB1 to PB3 runthrough a pixel 2, and a TFT Q, switches SW1 to SW3, a capacitanceelement C and a light emitting device LED are arranged in the pixel 2. Afirst terminal of the TFT Q, which is one of the drain or sourcethereof, is connected to the voltage supply line PB2, the switch SW3 isconnected between a second terminal of the TFT Q, which is the other ofthe drain or source thereof, and the light emitting device LED, and theswitch SW1 is connected between the second terminal of the TFT Q and thesignal line SL. A terminal of the light emitting device LED on the otherside of the switch SW3 is connected to the voltage supply line PB1. Theswitch SW2 is connected between the second terminal and gate of the TFTQ, and the capacitance element C is connected between the gate of theTFT Q and the voltage supply line PB3. Here, the switches SW1, SW2 andSW3 are controlled by the control lines CL1, CL2 and CL3, respectively.

[0069]FIG. 2 is a timing chart of a first example of an operationaccording to the first embodiment shown in FIG. 1. According to thisoperation example, in a first operation state (current retaining stateor row selection period), the switch SW1 is turned on by the controlline CL1, the switch SW2 is turned on by the control line CL2 and theswitch SW3 is turned off by the control line CL3. At this time, acurrent for providing an intended gray-scale is supplied to the signalline SL based on the current-brightness characteristic of the lightemitting device LED.

[0070] In the first operation state, the TFT Q operates in thesaturation region because the second terminal and gate thereof areshort-circuited by the switch SW2. Besides, since the switch SW3 hasbeen turned off, any current does not flow through the light emittingdevice LED, and the light emitting device LED does not operate (emitlight). The current supplied from the signal line SL flows into the TFTQ, and depending on the current/voltage characteristics of the TFT Q,the gate voltage of the TFT Q is set at a value for passing the currentacross the drain and source thereof. At this time, the current from thesignal line SL is not supplied to the capacitance of the light emittingdevice LED, and therefore, the gate voltage of the TFT Q is quickly setat the value for passing the current from the signal line SL across thedrain and source of the TFT Q.

[0071] The subsequent second operation state (current supplying state)is a state where a row other than that including the shown pixel in thedisplay device is selected, in which the switch SW1 is turned off by thecontrol line CL1, the switch SW2 is turned off by the control line CL2and the switch SW3 is turned on by the control line CL3.

[0072] In the second operation state, the gate voltage of the TFT Q isheld by the capacitance element C at the value in the first operationstate. Thus, the TFT Q can supply the current supplied thereto from thesignal line SL in the first operation state to the light emitting deviceLED through the switch SW3, and the light emitting device LED operates(emits light) to provide a brightness at an intended gray-scale level.

[0073] According to this embodiment, the TFT Q in the pixel retains,depending on the current/voltage characteristics thereof, the gatevoltage value to flow a current through the TFT Q from the signal lineSL, and the TFT Q having retained the gate voltage value supplies thecurrent, whose value is the same of the current from the signal line SLin the retaining state, to the light emitting device LED. Therefore, thecurrent can be retained and supplied with high precision regardless ofthe current/voltage characteristics of the TFT Q.

[0074] For the operation shown in FIG. 2, since the control lines CL1and CL2 operate the same, they can be integrated into one control line.Furthermore, in the case where the switches are constituted by TFTs withthe types of conductivity differing between the switches SW1, SW2 andthe switch SW3, the control lines CL1 to CL3 can be integrated into onecontrol line.

[0075]FIG. 3 is a timing chart of a second operation example accordingto the first embodiment shown in FIG. 1. A difference between thisoperation example and the first operation example shown in FIG. 2 isthat, in the first operation state, the switch SW2 is turned off earlierthan the switch SW1. For such an operation, if the switch SW2 is anelement having a capacitance between the gate and the drain, such asTFT, a TFT having the source and the drain short-circuited can beconnected between the switch SW2 and the gate of the TFT Q as a dummyswitch.

[0076] In the operation example shown in FIG. 3, the control lines CL1and CL2 cannot be integrated. However, since the switches SW1 and SW3are reverse-acting switches, the switches SW1 and SW3 can be constitutedby the TFTs with different types of conductivity (polarity), whereby thecontrol lines CL1 and CL3 can be integrated.

[0077]FIG. 4 is a timing chart of a third operation example according tothe first embodiment shown in FIG. 1. According to this operationexample, in the first operation state (current retaining state or rowselection period), the shown pixel is selected, the switch SW1 is turnedon by the control line CL1, the switch SW2 is turned on by the controlline CL2 and the switch SW3 is turned off by the control line CL3. Thus,the same operation as the first operation example shown in FIG. 2 isconducted.

[0078] The subsequent second operation state (current supplying state)is a state where a row other than that including the pixel shown in FIG.1 is selected, in which the switch SW1 is turned off by the control lineCL1, the switch SW2 is turned off by the control line CL2 and the switchSW3 is turned on by the control line CL3.

[0079] In this state, the gate voltage of the TFT Q is identical to thatretained by the capacitance element C in the first operation state, andthe TFT Q supplies the current supplied thereto from the signal line SLin the first operation state to the light emitting device LED throughthe switch SW3 to cause the light emitting device LED to emit light witha brightness at an intended gray-scale level.

[0080] In the subsequent third operation state (current stop state), therow other than that including the shown pixel has been selected, and theswitch SW3 is turned off by the control line CL3 before the rowincluding the shown pixel is selected again. Thus, the supply of thecurrent to the light emitting device LED is stopped, and the lightemitting device LED stops the operation thereof (light emission).

[0081] In the third operation example, among the first to thirdoperation states, the light emitting device LED emits light in thesecond operation state, stops light emission for a short while in thefirst operation state and does not emit light in the third operationstate. Therefore, it is possible to cause the light emitting device LEDto emit light only for a fraction of one frame period. For example, ifthe light emitting device is caused to emit light for one third of oneframe period, three times the current is to be supplied thereto toprovide the same time-average brightness as in the case of lightemission for the whole period. If the current value is raised, a timerequired for charging a wiring capacitance such as signal line can bereduced, and the period of the first operation state required forretaining the current can be shortened. Therefore, this operationexample is ready for the increase in wiring capacitance due to higherdefinition and larger screen. In addition, since the light emittingdevice does not emit light in the third operation state in thisoperation example, the display operation is similar to that of CRTs andan afterimage is hard to remain, and thus, moving images can bedisplayed with high quality.

[0082] In driving according to this operation example, the switches SW1and SW2 operate the same, and thus, the control lines CL1 and CL2 can beintegrated.

[0083] The third and second operation examples can be combined with eachother. That is, the timing chart shown in FIG. 4 can be modified so thatthe switch SW2 is turned of f before the first operation state expires.

[0084] [Second Embodiment]

[0085]FIG. 5 is a circuit diagram showing a configuration of one pixelaccording to a second embodiment of this invention. As shown in FIG. 5,a signal line SL extending in a column direction, control lines CL1 toCL3 extending in a row direction and voltage supply lines PB1 to PB3 runthrough a pixel 2, and a TFT Q, switches SW1 to SW3, a capacitanceelement C and a light emitting device LED are arranged in the pixel 2. Afirst terminal of the TFT Q, which is one of the drain or sourcethereof, is connected to the voltage supply line PB2, the switch SW3 isconnected between a second terminal of the TFT Q, which is the other ofthe drain or source thereof, and the light emitting device LED, and theswitch SW1 is connected between the second terminal of the TFT Q and thesignal line SL. A terminal of the light emitting device LED on the otherside of the switch SW3 is connected to the voltage supply line PB1. Theswitch SW2 is connected between the signal line SL and the gate of theTFT Q, and the capacitance element C is connected between the gate ofthe TFT Q and the voltage supply line PB3. Here, the switches SW1, SW2and SW3 are controlled by the control lines CL1, CL2 and CL3,respectively.

[0086]FIG. 6 is a timing chart of a first operation example according tothis embodiment. According to this operation example, a first operationstate (current retaining state or row selection period) includes aprecharge (voltage application) period and a current writing period.Providing the precharge period and applying an appropriate voltageduring the precharging enable the first operation state to be shortened,in particular, in the case where a low current is to be retained in thepixel circuit.

[0087] According to the first operation example of this embodiment, inthe precharge period in the first operation state, the shown pixel 2 isselected, the switches SW1 and SW3 are turned off, and the switch SW2 isturned on to apply a precharge voltage to the capacitance element C andthe gate of the TFT Q through the signal line SL. Then, in the currentwriting period in the first operation state, as in the first embodiment,the switches SW1 and SW2 are turned on and the switch SW3 is turned offto apply to the capacitance element C and the gate of the TFT Q avoltage for passing the current supplied through the signal line SLacross the drain and source of the TFT Q, thereby retaining the current.

[0088] According to the operation example of the first embodiment, thevoltage is applied to the capacitance element C relying on the current,a low current value would be affected by the load of the signal line SLor the like, so that it would take a long time for the voltage appliedto the gate of the TFT Q and the capacitance element to be settled.Thus, a long first operation state would be needed. To the contrary,according to this operation example, the precharge period in the firstoperation state, in which the voltage is precharged in the gate of theTFT Q and the capacitance element C, is short. When the prechargedvoltage is changed to an appropriate voltage to be applied to the gateof the TFT Q and the capacitance element C during the current writingperiod, the current writing period can also be reduced. Thus, the firstoperation state (total of the precharge period and the current writingperiod) can be reduced.

[0089] A second operation state (current supplying state) is a statewhere a pixel in a row other than the shown row is selected, in which asin the first embodiment, the switches SW1 and SW2 are turned off and theswitch SW3 is turned on to supply the retained current from the TFT Q tothe light emitting device LED.

[0090] The precharging operation in this operation example can berealized by changing the signal applied to the pixel 2 through thesignal line SL without changing the timings of the switching operationsaccording to the first embodiment. However, according to the firstembodiment, if a voltage is applied to the gate of the TFT Q and thecapacitance element C through the signal line SL in the precharge periodin the first operation state, the voltage may be different from thevoltage applied to the signal line SL because a current path isestablished. To the contrary, according to the second embodiment, sinceonly the switch SW 2 is in on state in the precharge period in the firstoperation state, no current path is established during the precharging.Thus, a precise voltage can be advantageously precharged to the gate ofthe TFT Q and the capacitance element C.

[0091] In this operation example, the timing for switching the switchSW1 from the off state to the on state is modified. Modifying the secondand third operation examples of the first embodiment in this way canprovide the first embodiment with the advantages of the first operationexample of the second embodiment in addition to its original advantages.On the other hand, according to the second embodiment, all the operationexample of the first embodiment is possible, and the advantages thereofare also provided. Furthermore, as in the first embodiment, theconfiguration of the pixel 2 can be simplified by appropriatelyselecting the types of conductivity of the transistors and integratingthe control lines for the operations.

[0092] [Third Embodiment]

[0093]FIG. 7 is a circuit diagram showing a configuration of one pixelaccording to a third embodiment of this invention. As shown in FIG. 7, asignal line SL extending in a column direction, control lines CL1 to CL3extending in a row direction and voltage supply lines PB1 to PB3 and PB5run through a pixel 2, and a TFT Q1, a TFT Q2, switches SW1 to SW3, acapacitance element C and a light emitting device LED are arranged inthe pixel 2. The TFT Q1 and the TFT Q2 are connected in series, one ofthe drain and source of the TFT Q2, which is not connected to the TFTQ1, is connected to the voltage supply line PB2, the switch SW3 isconnected between the light emitting device LED and one of the drain andsource of the TFT Q1, which is not connected to the TFT Q2, and theswitch SW1 is connected between the signal line SL and the terminal ofthe TFT Q1, which is not connected to the TFT Q2. A terminal of thelight emitting device LED on the other side of the switch SW3 isconnected to the voltage supply line PB1. The switch SW2 is connectedbetween the gate of the TFT Q1 and the terminal thereof, which is notconnected to the TFT Q2, the capacitance element C is connected betweenthe gate of the TFT Q1 and the voltage supply line PB3, and the gate ofthe TFT Q2 is connected to the voltage supply line PB5. Here, theswitches SW1, SW2 and SW3 are controlled by the control lines CL1, CL2and CL3, respectively.

[0094] According to the third embodiment, the TFT Q2, which is biased bythe voltage supply line PB5, is provided. Therefore, the TFT Q1 and theTFT Q2 are cascode-connected to each other and can be both made tooperate in the saturation region. Thus, the drain bias dependency of thecurrent/voltage characteristic of TFT Q1 in the saturation region can beimproved.

[0095] According to the third embodiment, the TFT Q2, which is biased bythe voltage supply line PB5, is provided. Therefore, the TFT Q1 and theTFT Q2 are cascode-connected to each other and can be both made tooperate in the saturation region. Thus, the drain bias dependency of theTFT Q1 in the saturation region can be improved.

[0096] The operation of the pixel in the third embodiment is the same asin the first embodiment except for the operation of the TFT Q2, and theadvantages of the operation examples in the first embodiment can beprovided. Furthermore, in this embodiment, changing the connection ofthe switches can realize the same operation as in the second embodiment,and the advantages of the operation example thereof can be provided.

[0097] [Fourth Embodiment]

[0098]FIG. 8 is a circuit diagram showing a configuration of one pixelaccording to a fourth embodiment of this invention. As shown in FIG. 8,a signal line SL extending in a column direction, control lines CL1 toCL4 extending in a row direction and voltage supply lines PB1 to PB4 runthrough a pixel 2, and a TFT Q, switches SW1 to SW4, a capacitanceelement C and a light emitting device LED are arranged in the pixel 2. Afirst terminal of the TFT Q, which is one of the drain or sourcethereof, is connected to the voltage supply line PB2, the switch SW3 isconnected between a second terminal of the TFT Q, which is the other ofthe drain or source thereof, and the light emitting device LED, and theswitch SW1 is connected between the second terminal of the TFT Q and thesignal line SL. A terminal of the light emitting device LED on the otherside of the switch SW3 is connected to the voltage supply line PB1. Theswitch SW4 has one terminal connected between the light emitting deviceLED and the switch SW3 and the other terminal connected to the voltagesupply line PB4. The switch SW2 is connected between the second terminaland gate of the TFT Q, and the capacitance element C is connectedbetween the gate of the TFT Q and the voltage supply line PB3. Here, theswitches SW1, SW2, SW3 and SW4 are controlled by the control lines CL1,CL2, CL3 and CL4, respectively.

[0099]FIG. 9 is a timing chart of an operation example according to thefourth embodiment of this invention shown in FIG. 8. According to thisoperation example, in a first operation state (current retaining stateor row selection period), the shown pixel is selected, the switch SW1 isturned on by the control line CL1, the switch SW2 is turned on by thecontrol line CL2, and the switches SW3 and SW4 stay of f and on underthe action of the control lines CL3 and CL4, respectively. In thisstate, as in the case of the circuit of the first embodiment, a voltageof the capacitance C and the gate of the TFT Q is set at a value forpassing the current supplied through the signal line SL across the drainand source of the TFT Q, and a voltage is applied to one terminal of thelight emitting device LED from the voltage supply line PB4 through theswitch SW4. The voltage applied to the light emitting device LED fromthe voltage supply line PB4 should be at a level not causing the lightemitting device LED to emit light.

[0100] The subsequent second operation state (current supplying state)is a state where a row other than that including the pixel shown in FIG.8 is selected, in which the switch SW1 is turned off by the control lineCL1, the switch SW2 is turned off by the control line CL2, the switchSW3 is turned on by the control line CL3, and the switch SW4 is turnedoff by the control line CL4.

[0101] In this state, the gate voltage of the TFT Q is identical to thatretained by the capacitance element C in the first operation state, andthe TFT Q supplies the current supplied thereto from the signal line SLin the first operation state to the light emitting device LED to causethe light emitting device LED to emit light with a brightness at anintended gray-scale level.

[0102] In the subsequent third operation state (current stop state), therow other than that including the shown pixel has been selected, and theswitch SW3 is turned off by the control line CL3 and the switch SW4 isturned on by the control line CL4 before the row including the shownpixel is selected again. Thus, the supply of the current to the lightemitting device LED is stopped, and charges stored in the light emittingdevice LED are rapidly removed, so that the light emitting device LEDstops the operation thereof (light emission).

[0103] This operation example is essentially the same as the thirdoperation example according to the first embodiment shown in FIG. 4.However, since the charges stored in the light emitting device LED areforcedly removed by the action of the switch SW4, the light emission bythe light emitting device can be stopped simultaneously with stoppingthe supply of the current thereto, so that the light emission period ofthe light emitting device can be controlled with higher precision. Here,for example, the voltage applied from the voltage supply line PB4 may beset at the same value as the voltage applied from the voltage supplyline PB1. In such a case, one terminal of the switch SW4 can beconnected to the voltage supply line PB1, rather than to the voltagesupply line PB4. Then, the voltage supply line PB4 is not necessary, sothat the configuration of the pixel 2 can be simplified.

[0104] In the operation example shown in FIG. 9, while the switches SW3and SW4 are reverse-acting switches in the operation example shown inFIG. 9, a modification can be made to the switch SW4 so that it stays ononly for a certain period from the beginning of the third operationstate.

[0105] Furthermore, in the fourth embodiment, an operation similar tothe second and third operation examples in the first embodiment arepossible.

[0106] Not only the first embodiment, but also each the second and thirdembodiment performs an operation as that of the fourth embodiment byadding the fourth switch and the fourth control line in the fourthembodiment thereto, respectively. In such cases, the light emission timeof the light emitting device can be controlled more accurately withoutloss of the advantages inherent in the embodiments and there respectiveoperations.

[0107] As described in detail with regard to the first embodiment, forthe operations in the first to fourth embodiments, the configuration ofthe pixel 2 can be simplified by appropriately selecting the types ofconductivity of the transistors and integrating the control lines.Furthermore, for example, the configuration of the pixel can besimplified by connecting a terminal of the capacitance element C on theother side of the retaining node to the voltage supply line PB1 or PB2,so that the voltage supply line PB3 can be removed. Besides, the valueof the voltage applied to the voltage supply line PB3 in the first andsecond operation state can be changed to change the current supplied tothe light emitting device. For example, if the voltage applied to thevoltage supply line PB3 in the second operation state is shifted fromthe voltage value in the first operation state to a level that causesthe TFT Q to be turned off, the gate voltage of the TFT Q is alsoshifted by the same amount on a boot effect, and thus, the current canbe prevented from flowing. Thus, a black state can be readily insertedfor improving the moving images display.

EXAMPLES

[0108] Now, examples of the present invention will be described indetail with reference to the drawings. In the following, the descriptionwill be made with regard to a light emitting device. However, it isillustrative only, and this invention can be applied to any typicalcurrent load element.

First Example

[0109]FIG. 10 shows a configuration of one pixel according to a firstexample of this invention. Here, all the pixels in the followingexamples are the pixel located in the Kth row and Mth column in thedisplay apparatus shown in FIG. 37. A signal line SL (#M), a powersupply line VCC, a ground line GND, a voltage supply line VS1, a controlline CL1 (#K) and a control line CL3 (#K) run through a pixel 2according to the first example of this invention, and a p-TFT Qp,switches SW1 to SW3, a capacitance element C and a light emitting deviceLED are arranged in the pixel 2. The source of the p-TFT Qp is connectedto the power supply line VCC, and the drain thereof is connected to oneend each of the switches SW1 to SW3, respectively. The other end of theswitch SW1 is connected to the signal line SL (#M), the other end of theswitch SW2 is connected to the gate of the p-TFT Qp, and the other endof the switch SW3 is connected to an anode of the light emitting deviceLED. The switches SW1 and SW2 are controlled by a signal in the controlline CL1 (#K), and the switch SW3 is controlled by a signal in thecontrol line CL3 (#K). A cathode of the light emitting device LED isconnected to the ground line GND, and one end of the capacitance elementC is connected to the gate of the p-TFT Qp and the other end thereof isconnected to the voltage supply line VS1. The voltage of the voltagesupply line VS1 should be kept constant.

[0110] An operation according to this example will be described below.FIG. 11 shows a first operation state in this example, FIG. 12 shows asecond operation state, and FIG. 13 is a timing chart of the operation.

[0111] The first operation state (current retaining state or rowselection period) in this operation example is a state where the Kth rowin the display device is selected, in which the switches SW1 and SW2 areturned on by the control line CL1 (#K), and the switch SW3 is turned offby the control line CL3 (#K). Besides, a current for providing anintended gray-scale is supplied to the signal line SL (#M) based on thecurrent-brightness characteristic of the light emitting device LED. Thatis, as shown in FIG. 11, a current I flows from the power supply lineVCC to the signal line SL (#M) through the p-TFT Qp.

[0112] In the first operation state, the p-TFT Qp operates in thesaturation region because the drain and the gate thereof isshort-circuited by the switch SW2. Besides, since the switch SW3 hasbeen turned off, any current does not flow through the light emittingdevice LED, and the light emitting device LED does not operate (emitlight). The current supplied from the signal line SL (#M) flows into thep-TFT Qp, and depending on the current/voltage characteristics of thep-TFT Qp, the gate voltage of the p-TFT Qp is set at a value for passingthe current across the drain and source thereof. At this time, thecapacitance of the light emitting device LED is independent of theoperation of passing the current through the p-TFT Qp, and needs not tobe charged or discharged by the current from the signal line SL (#M).Thus, the gate voltage of the p-TFT Qp is quickly set.

[0113] The second operation state (current supplying state) in thisexample is a state where a row other than the Kth row in the displaydevice is selected, in which the switches SW1 and SW2 are turned off bythe signal in the control line CL1 (#K), and the switch SW3 is turned onby the signal in the control line CL3 (#K).

[0114] In this operation state, the gate voltage of the p-TFT Qp is heldby the capacitance element C at the value in the first operation state,and thus, it is the same as the voltage between the gate and source ofthe p-TFT Qp in the first operation state. Since the p-TFT Qp suppliesthe current supplied thereto from the signal line SL (#M) in the firstoperation state to the light emitting device LED through the switch SW3,the light emitting device LED operates (emits light) to provide abrightness at an intended gray-scale level. That is, at this time, asshown in FIG. 12, the same current I as in the case shown in FIG. 11flows from the power supply line VCC to the ground line GND through thep-TFT Qp and the light emitting device LED. In this first operationexample, the same TFT serves both to retain the current and to supplythe current as described above. Therefore, the current can be retainedand supplied with high precision.

Second Example

[0115]FIG. 14 is a circuit diagram showing a configuration of one pixelaccording to a second example of this invention. The second example isthe same as the first example except that the channel type of the TFTfor supplying the current is changed from the p-channel type to then-channel type. That is, an n-channel type TFT (n-TFT, hereinafter) isused instead of the p-TFT in the first example. A signal line SL (#M), apower supply line VCC, a ground line GND, a voltage supply line VS1, acontrol line CL1 (#K) and a control line CL3 (#K) run through a pixel 2according to the second example of this invention, and an n-TFT Qn,switches SW1 to SW3, a capacitance element C and a light emitting deviceLED are arranged in the pixel 2. The source of the n-TFT Qn is connectedto the ground line GND, and the drain thereof is connected to one endeach of the switches SW1 to SW3, respectively. The other end of theswitch SW1 is connected to the signal line SL (#M), the other end of theswitch SW2 is connected to the gate of the n-TFT Qn, and the other endof the switch SW3 is connected to a cathode of the light emitting deviceLED. The switches SW1 and SW2 are controlled by a signal in the controlline CL1 (#K), and the switch SW3 is controlled by a signal in thecontrol line CL3 (#K). An anode of the light emitting device LED isconnected to the power supply line VCC, and one end of the capacitanceelement C is connected to the gate of the n-TFT Qn and the other endthereof is connected to the voltage supply line VS1. The voltage of thevoltage supply line VS1 is kept constant.

[0116] In this example, the control timing chart is the same as that inthe first example shown in FIG. 13, and the circuit according to thisexample operates the same and has the same advantage as in the firstexample.

Third Example

[0117]FIG. 15 is a circuit diagram showing a configuration of a pixelaccording to a third example of this invention, and FIG. 16 is a timingchart of an operation thereof.

[0118] A signal line SL (#M), a power supply line VCC, a ground lineGND, a voltage supply line VS1 and a control line CL1 (#K) run through apixel 2 according to this example, and a p-TFT Qp1, a p-TFT Qp2, ann-TFT Qn1, an n-TFT Qn2, a capacitance element C and a light emittingdevice LED are arranged in the pixel 2. In this example, the n-TFT Qn1,the n-TFT Qn2 and the p-TFT Qp2 serve as the switches SW1, SW2 and SW3in the first example, respectively (the p-TFT Qp1 serves as the p-TFT Qpin the first example). The operation according to the timing chart shownin FIG. 16 is the same as in the first example. According to thearrangement in this example, the control lines can be united into one.

Fourth Example

[0119]FIG. 17 is a circuit diagram showing a configuration of a pixelaccording to a fourth example of this invention, and FIG. 18 is a timingchart of an operation thereof.

[0120] A signal line SL (#M), a power supply line VCC, a ground lineGND, a voltage supply line VS1, a control line CL1 (#K) and a controlline CL2 (#K) run through a pixel 2 according to this example, and ap-TFT Qp1, a p-TFT Qp2, an n-TFT Qn1, an n-TFT Qn2, a capacitanceelement C and a light emitting device LED are arranged in the pixel 2.This example is different from the third example in that the controlline CL2 (#K) is additionally provided and the gate of the n-TFT Qn2 iscontrolled by the control line CL2 (#K). The operation according to thetiming chart shown in FIG. 18 is essentially the same as in the thirdexample (see FIG. 16). In this example, however, as shown in the timingchart shown in FIG. 18, the n-TFT Qn2 is turned off earlier by thecontrol line CL2 (#K), and then, the p-TFT Qp2 is turned on and then-TFT Qn1 is turned off by the control line CL1 (#K). According to suchan operation, a noise caused by the on/off operation of the p-TFT Qp2and n-TFT Qn1 can be prevented from being transmitted to the gate of thep-TFT Qp1. Thus, a more precise current can be supplied to the lightemitting device LED from the p-TFT Qp1.

Fifth Example

[0121]FIG. 19 is a circuit diagram showing a configuration of a pixelaccording to a fifth example of this invention, and FIG. 20 is a timingchart of an operation thereof.

[0122] A signal line SL (#M), a power supply line VCC, a ground lineGND, a power supply line VS1, a control line CL1 (#K), a control lineCL2 (#K) and a control line CL2B (#K) run through a pixel 2 according tothis example, and a p-TFT Qp1, a p-TFT Qp2, an n-TFT Qn1, an n-TFT Qn2,an n-TFT Qn3, a capacitance element C and a light emitting device LEDare arranged in the pixel 2. This example is different from the fourthexample (see FIG. 17) in that the control line CL2B (#K) and the n-TFTQn3 controlled by the control line CL2B (#K) are additionally provided.The n-TFT Qn3 has the source and the drain short-circuited and anappropriate ratio (W/L) between a gate length (L) and gate width (W)thereof with respect to the ratio of the n-TFT Qn2, and is connectedbetween the gate of the p-TFT Qp1 and the drain (or source) of then-TFTQn2. Since the n-TFT Qn2 has a capacitance (capacitance between the gateand the drain (or source)), when the n-TFT Qn2 is shifted from the onstate to the off state, the charges stored therein are moved and thegate potential of the p-TFT Qp1 is disturbed. The n-TFT Qn3 is intendedto cancel the movement of the charges for compensating for a voltageerror at the gate of the p-TFT Qp1. And, the n-TFT Qn3 has a capacitanceequivalent to that between the gate and the drain (or source) of then-TFT Qn2, and is controlled by the control line CL2B (#K) that carriesan inversion signal of the signal in the control line CL2 (#K) for then-TFT Qn2. In most cases, the ratio between the gate length and gatewidth of the n-TFT Qn3 is set at one half of that of the n-TFT Qn2.However, the ratio may vary with a timing condition or the like.According to this example including the n-TFT Qn3, a more precisecurrent can be supplied to the light emitting device LED from the p-TFTQp1.

Sixth Example

[0123] A sixth example is equivalent to the third example (see FIG. 15)having the channel types of all the TFTs being inverted. Therefore, theoperation timing chart in this example is equivalent to that in thethird example shown in FIG. 16 having the signals of the control linesCL1 (#K) and CL1 (#(K+1)) being inverted.

Seventh Example

[0124] A seventh example is equivalent to the fourth example (see FIG.17) having the channel types of all the TFTs being inverted. Therefore,the operation timing chart in this example is equivalent to that in thefourth example shown in FIG. 18 having the signals of the control linesCL1 (#K), CL1 (#(K+1)), CL2 (#K) and CL2 (#(K+1)) being inverted.

Eighth Example

[0125] An eighth example is equivalent to the fifth example (see FIG.19) having the channel types of all the TFTs being inverted. Therefore,the operation timing chart in this example is equivalent to that in thefifth example shown in FIG. 20 having the signals of the control linesCL1 (#K), CL1 (#(K+1)), CL2 (#K), CL2 (#(K+1)), CL2B (#K) and CL2B(#(K+1)) being inverted.

Ninth Example

[0126]FIG. 21 is a timing chart of an operation according to a ninthexample of this invention. The configuration of the pixel of the displaydevice used in this example is the same as that in the first exampleshown in FIG. 10.

[0127] The first operation state (current retaining state or rowselection period) in this example is a state where the Kth row in thedisplay device is selected, in which the switches SW1 and SW2 are turnedon by the control line CL1 (#K), and the switch SW3 is turned off by thecontrol line CL3 (#K). Besides, a current for providing an intendedgray-scale is supplied to the signal line SL (#M) based on thecurrent-brightness characteristic of the light emitting device LED.

[0128] The operation in the first operation state is the same as that inthe first example described with reference to FIGS. 10 to 13, andtherefore, detailed description thereof is omitted.

[0129] The second operation state (current supplying state) in thisexample is a state where a row other than the Kth row in the displaydevice is selected, in which the switches SW1 and SW2 are turned off bythe control line CL1 (#K), and the switch SW3 is turned on by thecontrol line CL3 (#K).

[0130] In the second operation state, the gate voltage of the p-TFT Qpis held by the capacitance element C at the value in the first operationstate, and thus, the voltage across the gate and source of the p-TFT Qpis the same as that in the first operation state. Since the p-TFT Qpsupplies the current supplied thereto from the signal line SL (#M) inthe first operation state to the light emitting device LED through theswitch SW3, the light emitting device LED operates (emits light) toprovide a brightness at an intended gray-scale level.

[0131] The third operation state (current stop state) in this examplecorresponds to a part of the period of the second operation state beforeentering into the first operation state, in which the switch SW3 isturned off by the control line CL2 (#K) while the switches SW1 and SW2are held in the off state by the control line CL1 (#K). During theperiod, since the switch SW3 is in the off state, any current is notsupplied to the light emitting device LED, and the light emitting deviceLED does not operate (emit light).

[0132] According to this example, in addition to the advantages of thecapability of quickly retaining a current and supplying the retainedcurrent to the light emitting device LED with high precision, which areattained by the first to eighth examples, the following advantage can beexpected. In this example, among the first to third operation states,the light emitting device LED emits light in the second operation state,stops light emission for a short while in the first operation state anddoes not emit light in the third operation state. Therefore, thetime-average brightness of the display device is T2/(T1+T2+T3) times thebrightness in the second operation state, where T1 denotes a period ofthe first operation state, T2 denotes a period of the second operationstate and T3 denotes a period of the third operation state. Assumingthat one frame period, which is the product of the selection time andthe number of stages (rows) to be controlled, is denoted by T, andT1=0.005T, T2=0.25T and T3=0.745T, for example, the brightness of thedisplay device is 0.25 times the brightness in the second operationstate. Accordingly, the brightness of the light emitting device LED inthe second operation state is required to be about four times higherthan that in the second operation state of the examples not having thirdoperation state. If the current-brightness characteristic of the lightemitting device LED exhibits a proportionality, the four times largercurrent is needed. According to this example, due to the presence of thethird operation state, the current passing through the light emittingdevice LED can be larger compared with the other examples. Thus, a timerequired for charging a wiring capacitance such as signal line can bereduced, and the period of the first operation state required forretaining the current can be shortened. Therefore, this example is readyfor the increase in wiring capacitance and the reduction of theselection time due to higher definition and larger screen. In addition,since the light emitting device LED does not emit light in the thirdoperation state in this example, the display operation is similar tothat of CRTs and an afterimage is hard to remain, and thus, movingimages can be displayed with high quality.

Tenth Example

[0133]FIG. 22 is a circuit diagram showing a configuration of a pixelaccording to a tenth example of this invention. A signal line SL (#M), apower supply line VCC, a ground line GND, a voltage supply line VS1, acontrol line CL1 (#K) and a control line CL3 (#K) run through a pixel 2according to this example, and a p-TFT Qp1, a p-TFT Qp2, an n-TFT Qn1,an n-TFT Qn2, a capacitance element C and a light emitting device LEDare arranged in the pixel 2. The pixel 2 in this example is equivalentto the pixel in the third example (see FIG. 15) additionally having thecontrol line CL3 (#K), which controls the p-TFT Qp2. FIG. 23 is a timingchart of an operation according to this example, which is equivalent tothat in the ninth example shown in FIG. 21 having the signals of thecontrol lines CL3 (#K) and CL3 (#(K+1)) being inverted. The circuit inthis example operates the same as in the ninth example.

Eleventh Example

[0134]FIG. 24 is a circuit diagram showing a configuration of a pixelaccording to an eleventh example of this invention, and FIG. 25 is atiming chart of an operation thereof. A signal line SL (#M), a powersupply line VCC, a ground line GND, a voltage supply line VS1, a controlline CL1 (#K), a control line CL2 (#K) and a control line CL3 (#K) runthrough a pixel 2 according to this example, and a p-TFT Qp1, a p-TFTQp2, an n-TFT Qn1, an n-TFT Qn2, a capacitance element C and a lightemitting device LED are arranged in the pixel 2. The pixel 2 in thisexample is equivalent to the pixel in the tenth example (see FIG. 22)additionally having the control line CL2 (#K), which controls the n-TFTQn2.

[0135] The operation according to the timing chart shown in FIG. 25 is acombination of the operation according to the tenth example shown inFIG. 23 and the operation according to the fourth example shown in FIG.17. That is, the n-TFT Qn2 is turned off earlier by the control line CL2(#K), and then, the n-TFT Qn1 is turned off by the control line CL1 (#K)and the p-TFT Qp2 is turned on by the control line CL3 (#K) to prevent anoise caused by the on/off operations of the p-TFT Qp2 and n-TFT Qn1from being transmitted to the gate terminal of the p-TFT Qp1. Then, theoperation shifts to the second operation state, and after that, thethird operation state is implemented (the p-TFT Qp2 is turned off).

Twelfth Example

[0136]FIG. 26 is a circuit diagram showing a configuration of a pixelaccording to a twelfth example of this invention, and FIG. 27 is atiming chart of an operation thereof. A signal line SL (#M), a powersupply line VCC, a ground line GND, a voltage supply line VS1, a controlline CL1 (#K), a control line CL2 (#K), a control line CL2B (#K) and acontrol line CL3 (#K) run through a pixel 2 according to this example,and a p-TFT Qp1, a p-TFT Qp2, an n-TFT Qn1, an n-TFT Qn2, an n-TFT Qn3,a capacitance element C and a light emitting device LED are arranged inthe pixel 2. The pixel according to this example is equivalent to thatin the eleventh example (see FIG. 24) additionally having the controlline CL2B (#K) and the n-TFT Qn3 controlled by the control line CL2B(#K), which is a combination of the eleventh example and the fifthexample (see FIG. 19).

[0137] The operation according to the timing chart shown in FIG. 27 is acombination of the operation according to the eleventh example shown inFIG. 25 and the operation according to the fifth example shown in FIG.20, which is characterized in that the n-TFT Qn3 absorbs a noise causedby switching of the n-TFT Qn2 controlled by the control line CL2 (#K).

[0138] As in the case of the second example for the first example, orthe sixth to eighth examples for third to fifth examples, for each ofthe ninth to twelfth example, alternative examples in which thepolarities of the TFTs are changed can be contemplated. In such cases,as in the case of the sixth to eighth examples for third to fifthexamples, if the switch TFTs are used, the polarities of the TFTs arechanged and the signals of the control lines are inverted.

Thirteenth Example

[0139]FIG. 28 is a circuit diagram showing a configuration of a pixelaccording to a thirteenth example of this invention. A signal line SL(#M), a power supply line VCC, a ground line GND, a voltage supply lineVS1, control lines CL1 (#K), CL2 (#K) and CL3 (#K) run through a pixel 2according to this example, and a p-TFT Qp, switches SW1 to SW3, acapacitance element C and a light emitting device LED are arranged inthe pixel 2. The source of the p-TFT Qp is connected to the power supplyline VCC. The switch SW3, which is controlled by the control line CL3(#K), is connected between the drain of the P-TFT Qp and an anode of thelight emitting device LED, and the switch SW1, which is controlled bythe control line CL1 (#K), is connected between the drain of the p-TFTQp and the signal line SL. A cathode of the light emitting device LED isconnected to the ground line GND. In addition, the switch SW2, which iscontrolled by the control line CL2 (#K), is connected between the signalline SL and the gate of the p-TFT Qp, and the capacitance element C isconnected between the gate of the p-TFT Qp and the voltage supply lineVS1.

[0140] An operation according to the thirteenth example will bedescribed below. FIG. 29 is a timing chart of an operation according tothis example.

[0141] The first operation state (current retaining state or rowselection period) in this example is a state where the Kth row isselected, and includes two periods. In the first period (prechargeperiod), the switch SW1 is turned off by the control line CL1 (#K), theswitch SW2 is turned on by the control line CL2 (#K) and the switch SW3is turned off by the control line CL3 (#K). During this period, anappropriate voltage is applied to the gate of the p-TFT Qp through thesignal line SL (#M). In the second period (current writing period), theswitch SW1 is turned on by the control line CL1 (#K), and the switchesSW2 and SW3 are not changed from the respective states in the firstperiod. During this period, a current corresponding to a gray-scalelevel is supplied to the p-TFT Qp through the signal line SL (#K), thegate voltage of the p-TFT Qp is set at a value for passing the currentacross the drain and source thereof, and the voltage is maintained(retained) in the capacitance element C. The current writing period isequivalent to the first operation state in the first to twelfthexamples.

[0142] The second operation state (current supplying state) in thisexample is a state where a row other than the Kth row in the displaydevice is selected, in which the switches SW1 and SW2 are turned off bythe signal in the control line CL1 (#K), and the switch SW3 is turned onby the signal in the control line CL3 (#K). In this operation state, asin the second operation state in the first to twelfth examples, thep-TFT Q supplies the current retained during the first operation stateto the light emitting device LED.

[0143] This example is characterized in that the first operation stateincludes the precharge period in which a voltage is applied to the gateof the p-TFT Q. Applying an appropriate precharge voltage to the gate ofthe p-TFT Q during the precharge period can provide a shortened currentwriting period only enough for correction. Thus, the period of the firstoperation state (total of the precharge period and the current writingperiod) can be shortened. While the first operation state including thesimilar precharge period can be implemented in the first to twelfthexamples, a current path exists during the precharge period. To thecontrary, in this example, since the switch SW1 stays off during theprecharge period, no current path exist, and the voltage can be appliedwith high precision.

[0144] Here, the arrangement according to the thirteenth example isimplemented by modifying the connection of the switch SW2 in thearrangement according to the first example. Therefore, the first totwelfth examples can be similarly modified by changing the position ofthe switch SW2 as in the thirteenth example. FIG. 30 shows such amodification of the third example (FIG. 15) implemented by modifying theconnection of the switch SW2 as in the example 13. These modifiedcircuits can perform the same operation as that of the first to twelfthexamples and the thirteenth example which has the precharge operation,with the advantages of those examples.

Fourteenth Example

[0145]FIG. 31 is a circuit diagram showing a configuration of a pixelaccording to a fourteenth example of this invention. A signal line SL(#M), a power supply line VCC, a ground line GND, voltage supply linesVS1,VS3, control lines CL1 (#K) and CL3 (#K) run through a pixel 2according to this example, and a p-TFT Qp1, a p-TFT Qp2, switches SW1 toSW3, a capacitance element C and a light emitting device LED arearranged in the pixel 2. The source of the p-TFT Qp1 is connected to thepower supply line VCC via the p-TFT Qp2. The switch SW3, which iscontrolled by the control line CL3 (#K), is connected between the drainof the P-TFT Qp1 and an anode of the light emitting device LED, and theswitch SW1, which is controlled by the control line CL1 (#K), isconnected between the drain of the p-TFT Qp1 and the signal line SL(#M). A cathode of the light emitting device LED is connected to theground line GND. In addition, the switch SW2, which is controlled by thecontrol line CL1 (#K), is connected between the gate and drain of thep-TFT Qp1, the capacitance element C is connected between the switch SW2and the voltage supply line VS1, and the gate of the p-TFT Qp2 isconnected to the voltage supply line VS3.

[0146] The operation in the fourteenth example is the same as that inthe first example. However, in this example, the p-TFT Qp2, which isbiased by the voltage supply line VS3, is provided. Therefore, forexample, the p-TFT Qp1 and the p-TFT Qp2 can be both made to operate inthe saturation region. Thus, the drain voltage dependency of thecurrent/voltage characteristic of p-TFT Qp1 in the saturation region canbe improved.

[0147] Here, the arrangement according to the fourteenth example isimplemented by adding the p-TFT Qp2 to the arrangement according to thefirst example. Therefore, the first to twelfth examples can be similarlymodified by adding the p-TFT to the arrangements thereof as in thefourteenth example. FIG. 32 shows such a modification of the tenthexample (FIG. 22) implemented by adding the p-TFT Qp3 thereto.Furthermore, the thirteenth example can be similarly modified by addinganother p-TFT to the arrangement thereof as in the fourteenth example.

Fifteenth Example

[0148]FIG. 33 is a circuit diagram showing a configuration of a pixelaccording to a fifteenth example of this invention, and FIG. 34 is atiming chart of an operation in this example. A signal line SL (#M), apower supply line VCC, a ground line GND, a voltage supply line VS1, avoltage supply line VS2, a control line CL1 (#K), a control line CL3(#K) and a control line CL4 (#K) run through a pixel 2 according to thefifteenth example of this invention, and a p-TFT Qp, switches SW1 toSW4, a capacitance element C and a light emitting device LED arearranged in the pixel 2. The source of the p-TFT Qp is connected to thepower supply line VCC, the switch SW3, which is controlled by thecontrol line CL3 (#K), is connected between the drain of the p-TFT Qpand an anode of the light emitting device LED, and the switch SW1, whichis controlled by the control line CL1, is connected between the drain ofthe p-TFT Qp and the signal line SL (#M). A cathode of the lightemitting device LED is connected to the ground line GND. The switch SW4,which is controlled by the control line CL4 (#K), is connected betweenthe anode of the light emitting device LED and the voltage supply lineVS2. The switch SW2, which is controlled by the control line CL1 (#K),is connected between the drain and gate of the p-TFT Qp, and thecapacitance element C is connected between the gate of the p-TFT Qp andthe voltage supply line VS1.

[0149] In the first operation state (current retaining state or rowselection period) in this operation example shown in FIG. 34, the Kthrow in the display device is selected, the switches SW1 and SW2 areturned on by the control line CL1 (#K), the switch SW3 is turned off bythe control line CL3 (#K), and the switch SW4 is turned on by thecontrol line CL4 (#K) (however, operation is possible regardless ofwhether the switch SW4 is in on or off state, in this operation state).Besides, a current for providing an intended gray-scale is supplied tothe signal line SL (#M) based on the current-brightness characteristicof the light emitting device LED. In the first operation state, thevoltage of the gate of the p-TFT Qp is the voltage which iscorresponding to the current flowing across the drain and the source ofthe p-TFT Qp. The current is provided through the signal line SL(#M).

[0150] The second operation state (current supplying state) in thisexample is a state where a row other than the Kth row in the displaydevice is selected, in which the switches SW1 and SW2 are turned off bythe control line CL1 (#K), the switch SW3 is turned on by the controlline CL3 (#K), and the switch SW4 is turned off by the control line CL4(#K). In the second operation state, the gate voltage of the p-TFT Qp isheld by the capacitance element C at the value in the first operationstate, and thus, the voltage across the gate and source of the p-TFT Qpis the same as that in the first operation state. Since the currentsupplied thereto from the signal line SL (#M) in the first operationstate is supplied to the light emitting device LED through the switchSW3, the light emitting device LED operates (emits light) to provide abrightness at an intended gray-scale level.

[0151] The third operation state (current stop state) in this example,in which a row other than the Kth row in the display device is selected,corresponds to a period in which the switch SW3 is turned off by thecontrol line CL3 (#K) and the switch SW4 is turned on by the controlline CL4 (#K) while the switches SW1 and SW2 are held in the off stateby the control line CL1 (#K). At the start of this operation state, theswitch SW3 is turned off and the switch SW4 is turned on, whereby anycurrent is not supplied to the light emitting device LED, and thevoltage VS3 is applied to the anode of the light emitting device. Whenthe voltage VS3 is lower than the emitting voltage of the light emittingdevice LED, the light emitting device LED instantaneously stops theoperation (light emission) at the start of the operation state.

[0152] As in the other examples, according to this example, a currentcan be retained quickly and the retained current can be supplied to thelight emitting device LED with high precision.

[0153] As in the ninth to twelfth examples, according to this example,the current passing through the signal line SL into the light emittingdevice LED can be increased. Thus, a time required for charging a wiringcapacitance such as signal line can be reduced, and the period of thefirst operation state required for retaining the current can beshortened. Therefore, this example is ready for the increase of thewiring capacitance element C and the reduction of the selection time dueto higher definition and larger screen.

[0154] In addition, according to this example, the switch SW4 isprovided and can be turned on to apply the voltage VS3 to the lightemitting device LED at the start of the third operation state, therebyinstantaneously stopping the light emission. In the ninth to twelfthexamples, even if the current path is interrupted by the switch SW3, acurrent is supplied to the light emitting device due to the chargesstored in the capacitance of the light emitting device itself. Thus, thelight emitting device continues to operate (emit light) until thevoltage across the capacitance is sufficiently reduced. This lightemission causes an error in determining the brightness of the displaydevice based on the brightness in the second operation state and theperiods of the respective operation states. On the other hand, accordingto this example, since the light emission can be stopped instantaneouslyby the switch SW4, the brightness of the display device can bedetermined with high precision based on the brightness in the secondoperation state and the periods of the first, second and third operationstates. Also, as in the ninth to twelfth examples, since the lightemission halts in the third operation state, the display operation issimilar to that of CRTs, and thus, moving images can be displayed withhigh quality.

[0155] Here, the arrangement according to the fifteenth example isimplemented by adding the switch SW4, the control line CL4 (#K) and thevoltage supply line VS2 to the arrangement according to the firstexample (FIG. 10). Therefore, the first to twelfth examples can besimilarly modified by adding the switch SW4, or TFT and its control lineto the arrangements thereof as in the fifteenth example. FIG. 35 showssuch a modification of the third example (FIG. 15) implemented by addingthe n-TFT Qn3 and voltage supply line VS2 thereto. FIG. 36 shows such amodification of the tenth example (FIG. 22) implemented by adding then-TFT Qn3 and the voltage supply line VS2 thereto. Furthermore, addingthe switch SW4 (or TFT serving as a switch) to the arrangementsaccording to the thirteenth and fourteenth examples can providemodifications with the characteristics of this example in addition tothe characteristics of the thirteenth and fourteenth examples,respectively.

[0156] In the fifteenth example, the voltage supply line VS2 is onlyneeded to feed a voltage for stopping the light emission momentarilywhen entering the third operation state. Therefore, for example, it maybe integrated with the ground line GND to simplify the configuration ofthe pixel 2 in this example.

Sixteenth Example

[0157] In the first to fifteenth examples, the voltage supply line VS1,which is connected to one terminal of the capacitance element having theother terminal thereof connected to the gate of the TFT, is assumed tobe kept at a constant voltage. Therefore, the power supply line VCC orground line GND may serve also as the voltage supply line VS1, and insuch a case, the configuration of the pixel can be simplified. The valueof the current to be supplied to the light emitting device can bechanged by varying the voltage value of the voltage supply line VS1 inthe first operation state from that in the other operation states.

[0158] For example, if the voltage of the voltage supply line VS1 isshifted from the value in the first operation state to a level thatcauses the TFT to be turned off, the TFT can be turned off on the booteffect. If such an operation is performed on the entire light emittingdisplay device or on each line, the entire light emitting display deviceor each line can be brought into the black state (a state where thelight emitting devices are not activated).

[0159] The preferred embodiments and examples have been described above.However, this invention is not limited thereto and can be appropriatelyaltered without departing the spirit and scope thereof. For example, asdescribed above, elements other than the light emitting device includingan inorganic EL and an organic EL device such as a light emitting diodemay be used, and a general current load element may be used. The thirdswitch (SW3), which is inserted in the current path of the lightemitting device, may be disposed on the side of the poser supply line(or ground line), rather than on the side of the driving transistor forthe light emitting device. Furthermore, while the fourth switch (SW4) isprovided only in the case where the third switch is turned off earlierin the examples, it may be provided in the display device in which thethird switch is turned off when the first switch is turned on.Furthermore, the switch used in this invention is not limited to the TFTswitch. The switch is essentially prescribed with regard to operationthereof. While the examples involving the simplified configuration havebeen described in the above-described examples, the transistor servingas the switch may have any polarity as far as it adequately operates.

[0160] A first advantage of this invention is that a precise current canbe supplied to the current load element. A first reason therefor is thatthe signal is supplied to the signal line via the current, and the sametransistor serves both to retain the current flowing through the signalline and to supply the current to the current load element, therebypreventing the performance of the current load element from beingaffected by the characteristic variation between the transistors. Asecond reason therefor is that the current from the signal line can beretained accurately because the current is retained in the state whereany current is not supplied to the current load element.

[0161] A second advantage is that a time required for retaining thecurrent is short, and a higher definition can be supported. This is dueto the fact that the switch between the transistor for retaining thecurrent and the current load element stays off during a period ofretaining the current, and thus, the retaining of the current can beconducted without being affected by the high load of the current loadelement (capacitance and resistance in parallel).

[0162] Furthermore, according to the example in which the switch SW2 isturned off earlier than the switch SW1, the noise caused when the switchSW1 is changed in its state can be prevented from being transmitted tothe gate of the TFT for driving the current load element. Thus, a highprecision current can be supplied to the current load element.

[0163] Furthermore, according to the example in which the switch SW2 isinterposed between the signal line and the gate of the transistor forsupplying a current, highly precise precharging operation can beperformed, and the period for retaining the current can be shortened.

[0164] Furthermore, according to the example in which the transistor isinterposed between the transistor for supplying a current and the powersupply line, the drain voltage dependency of the drain current of thetransistor for supplying a current can be improved by appropriatelybiasing the gate of the transistor. Thus, a high precision current canbe supplied to the current load element.

[0165] In the case where the current load element is the light emittingdevice, according to the example in which an operation state where anycurrent does not flow through the light emitting device is providedduring a period in which the pixel is deselected, the current to beretained can be increased, so that the current can be retained in ashorter time, and the operation becomes similar to that of CRTs, so thatan afterimage is hard to remain. Thus, moving images can be displayedwith higher quality.

What is claimed is:
 1. A current load device, comprising: a drivingtransistor having a source connected to a power supply line or groundline; a first switch connected between a signal line supplied with acurrent or voltage and a drain of said driving transistor; a secondswitch connected between said signal line or the drain of said drivingtransistor and a gate of said driving transistor; a capacitance elementhaving one terminal connected to a first voltage supply line and theother terminal connected to the gate of said driving transistor; and aserially-connected assembly of a current load element and a thirdswitch, the serially-connected assembly being connected between theground line or power supply line and the drain of said drivingtransistor.
 2. A current load device, comprising: a first transistorhaving a source connected to a power supply line or ground line and agate connected to a second voltage supply line; a driving transistorserially connected to said first transistor; a first switch connectedbetween a signal line supplied with a current or voltage and a drain ofsaid driving transistor; a second switch connected between said signalline or the drain of said driving transistor and a gate of said drivingtransistor; a capacitance element having one terminal connected to afirst voltage supply line and the other terminal connected to the gateof said driving transistor; and a serially-connected assembly of acurrent load element and a third switch, the serially-connected assemblybeing connected between the ground line or power supply line and thedrain of said driving transistor.
 3. The current load device accordingto claim 1, wherein in a first operation state, said first and secondswitches are turned on and said third switch is turned off to store agate voltage in accordance with current/voltage characteristics of saiddriving transistor in said capacitance element so that the currentflowing through said signal line flows between the drain and source ofsaid driving transistor without passing a current through said currentload element, and then in a second operation state, said first andsecond switches are turned off and the third switch is turned on to letsaid driving transistor supply the current whose value is the same thatof current having flowed through the signal line in said first operationstate to said current load element through said third switch.
 4. Thecurrent load device according to claim 1, wherein in a first operationstate, said first and second switches are turned on and said thirdswitch is turned off to apply the voltage applied to said signal line tothe gate of said driving transistor and said capacitance element withoutpassing a current through said current load element, and then a gatevoltage in accordance with current/voltage characteristics of saiddriving transistor is stored in said capacitance element so that thecurrent flowing through said signal line flows between the drain andsource of said driving transistor, and in a second operation state, saidfirst and second switches are turned off and the third switch is turnedon to let said driving transistor supply the current whose value is thesame that of current having flowed through the signal line in said firstoperation state to said current load element through said third switch.5. The current load device according to claim 1, wherein in a firstoperation state, said second switch between said signal line and saiddriving transistor is turned on and said first and third switches areturned off to apply the voltage applied to said signal line to the gateof said driving transistor and said capacitance element without passinga current through said current load element, and then, said first andsecond switches are turned on and said third switch is turned off tostore a gate voltage in accordance with current/voltage characteristicsof said driving transistor in said capacitance element so that thecurrent flowing through said signal line flows between the drain andsource of said driving transistor, and in a second operation state, saidfirst and second switches are turned off and the third switch is turnedon to let said driving transistor supply the current whose value is thesame that of current having flowed through the signal line in said firstoperation state to said current load element through said third switch.6. The current load device according to claim 3, wherein said thirdswitch is turned off before said first switch is turned on, and isturned on after said first switch is turned off.
 7. The current loaddevice according to claim 3, wherein said second switch is turned offbefore said first switch is turned off.
 8. The current load deviceaccording to claim 1, wherein said driving transistor is constituted bya thin film transistor (TFT).
 9. The current load device according toclaim 1, wherein the first, second and third switches are eachconstituted by a TFT.
 10. The current load device according to claim 1,wherein said first and second switches are TFTs having a same polarity,and said third switch is a TFT having a polarity opposite to that ofsaid first and second switches.
 11. The current load device according toclaim 1, wherein said first, second and third switches are controlled byone control line.
 12. The current load device according to claim 1,wherein said first and second switches are controlled by one controlline.
 13. The current load device according to claim 1, wherein saidfirst and third switches are controlled by one control line.
 14. Thecurrent load device according to claim 1, wherein a TFT which has thedrain and source short-circuited and operates oppositely to said secondswitch TFT is connected between said second switch TFT and said drivingtransistor TFT as a dummy switch.
 15. The current load device accordingto claim 14, wherein a ratio (W/L) between a length (L) and a width (W)of the gate of said dummy switch TFT is a half the ratio between thelength and the width of the gate of said second switch TFT.
 16. Thecurrent load device according to claim 1, further comprising a fourthswitch having one terminal connected between said third switch and saidcurrent load element and another terminal connected to a third voltagesupply line.
 17. The current load device according to claim 16, whereina value of a voltage applied to said third voltage supply line is lowerthan a voltage value for said current load element to start to operate.18. The current load device according to claim 17, wherein said fourthswitch is turned on when said third switch is turned off, wherebycharges stored in said current load element are forcedly removed and thecurrent flowing through said current load element is quickly stopped.19. The current load device according to claim 16, wherein said fourthswitch is constituted by a TFT.
 20. The current load device according toclaim 19, wherein said third and fourth switches are TFTs havingopposite polarities, whereby said third and fourth switches arecontrolled by one control line.
 21. The current load device according toclaim 1, wherein said first voltage supply line is the power supply lineor ground line.
 22. The current load device according to claim 1,wherein the voltage applied through said first voltage supply linediffers between said first operation state and said second operationstate.
 23. A method for driving a current load device, the current loaddevice being active-matrix driven and comprising a plurality of cellseach comprising a current load element, a driving transistor for drivingthe current load element and a retention capacitance element forretaining a voltage to be applied to said driving transistor, whereinsaid current load element has not been driven not only for the period inwhich said retention capacitance element is setting an appropriatevoltage level to be retained therein, but also for a part of the periodin which said retention capacitance element does not perform the settingoperation.
 24. The method for driving a current load device according toclaim 23, wherein the supply of the current to said current load elementis stopped before the operation for setting the voltage level for saidretention capacitance element.
 25. The method for driving a current loaddevice according to claim 23, wherein when the supply of the current tosaid current load element is stopped, charges stored in said currentload element is forcedly removed.
 26. The method for driving a currentload device according to claim 23, wherein when setting the voltagelevel for said retention capacitance element, said driving transistoroperates in a saturation region.
 27. The method for driving a currentload device according to claim 23, wherein when setting the voltagelevel for said retention capacitance element, the performance of passingthe current through said driving transistor is set after the performanceof supplying the voltage to said retention capacitance element and saiddriving transistor.
 28. The method for driving a current load deviceaccording to claim 23, wherein the operation for setting the voltagelevel for said retention capacitance element includes a period in whichthe current flowing through the signal line is passed across a drain andsource of said driving transistor.
 29. The method for driving a currentload device according to claim 23, wherein a performance of said currentload element is set based on two factors, a first factor being theperformance of said current load element in the case of being driven bysaid driving transistor, and a second factor being a ratio between aperiod in which said current load element operates and a period in whichsaid current load element does not operate.
 30. The current load deviceaccording to claim 1, wherein said current load element is a lightemitting device.
 31. The current load device according to claim 1,wherein said current load element is an organic EL device.
 32. Themethod for driving a current load device according to claim 23, whereinsaid current load element is a light emitting device.
 33. The method fordriving a current load device according to claim 23, wherein saidcurrent load element is an organic EL device.